good effort mate, for the light graffiti images use manual focus and find a point to focus on (normally if its dark then a mate holding a torch is a good bet) then when you do you graffiti do it in the same place then you will get it nice a sharp. Also use a decent tripod with self time to try to reduce camera shake by as much as possible.
for the city & traffic scene try using a small aperture (high f number ie 22) then you get nice star burst effects on any street lights etc.
Its mainly just a case of experimenting and practise, you can really go wrong with shooting digital, just keep trying and delete what you dont want.
i have put up a few of mine hope you dont mind.